Lithographic apparatus and gas lock funnel

Abstract

A lithographic apparatus comprises: a substrate table; a projection system; a gas lock funnel; and a membrane. The substrate table is configured to hold a substrate. The projection system comprises optics configured to project radiation onto a target portion of the substrate. The membrane is supported by the gas lock funnel and arranged between the projection system and the substrate table. The gas lock funnel may defines a nozzle that is arranged to provide a gas in the vicinity of the membrane such that on the projection system side of the membrane the gas is directed preferentially towards a peripheral portion of the gas lock funnel. A final optical element of the projection system may have a central aperture and the nozzle may be arranged to direct the gas preferentially away from the central aperture in the final optical element.

Description

LITHOGRAPHIC APPARATUS AND GAS LOCK FUNNELCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 23197801.6 which was filed on 15 September 2023 and US application 63 / 655,324 which was filed on 3 June 2024 and which are incorporated herein in its entirety by reference.FIELD

[0002] The present disclosure relates to a lithographic apparatus comprising a gas lock funnel that forms part of a dynamic gas lock. The lithographic apparatus may be an extreme ultraviolet (EUV) lithographic apparatus. The present disclosure also relates to a gas lock funnel for use in a lithographic apparatus.BACKGROUND

[0003] A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may, for example, project a pattern at a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate.

[0004] To project a pattern on a substrate a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features which can be formed on the substrate. A lithographic apparatus, which uses extreme ultraviolet (EUV) radiation, having a wavelength within the range 4-20 nm, for example 6.7 nm or 13.5 nm, may be used to form smaller features on a substrate than a lithographic apparatus which uses, for example, radiation with a wavelength of 193 nm.

[0005] A lithographic apparatus comprises projection optics configured to project radiation onto a target portion of a substrate (for example a resist-coated silicon wafer) that is supported by a substrate table (also referred to as a wafer stage). In particular, the projection optics are arranged to form an image of a patterning device (a mask or reticle) on the target portion of the substrate. The lithographic apparatus may additionally comprise a gas lock funnel that defines an exit aperture or exit window of a housing of the lithographic apparatus, for example at the projection optics to allow the radiation to exit the housing of the projection optics and be projected onto the target portion of the substrate. The gas lock funnel may form part of a dynamic gas lock. A membrane may be supported by the gas lock funnel and may be arranged between the projection system and the substrate table. The membrane can act to suppress out of band radiation (i.e. radiation that is not being used for lithographic exposure) from being transmitted to the substrate and to protect the projection system optics from resist outgassing.

[0006] It may be desirable to provide an alternative gas lock funnel and / or lithographic apparatus that at least partially addresses one or more problems with existing arrangements, whether expressly identified herein or not.SUMMARY

[0007] According to a first aspect of the present disclosure there is provided a lithographic system comprising: an optical system comprising optics configured to project radiation onto a target portion of a substrate or to project radiation onto a patterning device; a second system; and a gas lock funnel arranged between the optical system and the second system; wherein the gas lock funnel defines a nozzle that is arranged to provide a gas to an interior of the gas lock funnel such that on the optical system side of the gas lock funnel the gas is directed preferentially away from optical elements of the optical system.

[0008] A lithographic system may comprise: a radiation source and a lithographic apparatus.

[0009] It will be appreciated that the gas being directed preferentially away from optical elements of the optical system is intended to mean that at least a majority of the gas is directed towards something which is not used to direct the radiation (towards a substrate or a patterning device). That is, preferably at least 50% of the gas may be directed so as not to impinge on the optical elements of the optical system, more preferably at least 75% and even more preferable at least 85% of the gas may be directed so as not to impinge on the optical elements of the optical system. Optical elements of the optical system may include, for example, mirrors, sensors and so on.

[0010] The lithographic system may be an EUV lithographic system and the optical elements of the optical system may be arranged to reflect EUV radiation. In some embodiments, the nozzle may be arranged such that the gas is directed preferentially towards a surface which is non-EUV reflective. The lithographic system according to the first aspect is advantageous as it reduces the amount of debris that is carried by the gas flow towards optical elements of the optical system.

[0011] In some embodiments, the second system may comprise a substrate table configured to hold a substrate and the optical system may comprise a projection system comprising optics configured to project radiation onto the target portion of the substrate.

[0012] In some embodiments, the second system may comprise a radiation source and the optical system may comprise an illuminator system configured to project radiation onto a patterning device.

[0013] The radiation source may, for example, comprise a laser-produced plasma (LPP) radiation source.

[0014] According to a second aspect of the present disclosure there is provided a lithographic apparatus comprising: a substrate table configured to hold a substrate; a projection system comprising optics configured to project radiation onto a target portion of the substrate; and a gas lock funnel arranged between the projection system and the substrate table; wherein the gas lock funnel defines a nozzle that is arranged to provide a gas to an interior of the gas lock funnel such that on the projectionsystem side of the gas lock funnel the gas is directed preferentially away from optical elements of the projection system.

[0015] The projection system may alternatively be referred to as an optical system in the following.

[0016] It will be appreciated that the gas being directed preferentially away from optical elements of the projection system is intended to mean that at least a majority of the gas is directed towards something which is not used to direct the radiation towards the substrate. That is, preferably at least 50% of the gas may be directed so as not to impinge on the optical elements of the projection system, more preferably at least 75% and even more preferable at least 85% of the gas may be directed so as not to impinge on the optical elements of the projection system. Optical elements of the projection system may include, for example, mirrors, sensors and so on.

[0017] The lithographic apparatus may be an EUV lithographic apparatus and the optical elements of the projection system may be arranged to reflect EUV radiation. In some embodiments, the nozzle may be arranged such that the gas is directed preferentially towards a surface which is non-EUV reflective.

[0018] In some embodiments, the nozzle may be arranged such that the gas is directed preferentially towards a peripheral portion of the gas lock funnel. The gas lock funnel may be shaped such that a taper of the gas lock funnel is generally matched to a converging beam of (EUV) radiation as it is focused on the substrate. Therefore, by directing the gas preferentially towards a peripheral portion of the gas lock funnel, the gas will be directed away from, or at worst to a peripheral portion of, a last optical element of the projection system.

[0019] Additionally or alternatively, in some embodiments, the nozzle may be arranged such that the gas is directed preferentially towards a wall of the projection system. Here a wall of the projection system may include a wall of a housing of the projection optics. Again, the beam of (EUV) radiation as it is focused on the substrate is not incident on the walls of the projection system. Therefore, by directing the gas preferentially towards a wall of the projection system, the gas will be directed away from the optical elements of the projection system.

[0020] In use, the gas lock funnel forms part of a dynamic gas lock. The gas lock funnel may be considered to define an exit aperture or exit window of a housing of the projection optics to allow the radiation to exit the housing of the projection optics and be projected onto the target portion of the substrate. The membrane acts to suppress out of band radiation (i.e. radiation that is not being used for lithographic exposure) from being transmitted to the substrate and to protect the projection system optics from resist outgassing.

[0021] The lithographic apparatus may further comprise a source of a radiation beam. The radiation beam may comprise desired radiation having a first wavelength / wavelength range, and undesired radiation having a second wavelength / wavelength range. The first wavelength / wavelength range may comprise EUV radiation. The second wavelength / wavelength range may comprise deep ultraviolet (DUV) radiation and / or infrared (IR) radiation.

[0022] In some embodiments, the lithographic apparatus may comprise a membrane supported by the gas lock funnel and arranged between the projection system and the substrate table. Such an arrangement may be referred to as a closed configuration. The membrane is provided in the optical path of the incoming radiation. The membrane may act to filter infrared (IR) and deep ultraviolet (DUV) radiation while transmitting EUV radiation. The nozzle defined by the gas lock funnel may comprise two portions: a first portion arranged to deliver gas to the projection system side of the membrane and a second portion arranged to deliver gas to the substrate table side of the membrane. The gas flow from the first portion of the nozzle acts as a particle stopping flow to protect the membrane from particles within the projection optics housing impinging on the membrane.

[0023] In use, the membrane can rupture, which can cause debris from the membrane. This debris can be transported to the optical elements in the projection system by the gas flow from the nozzle. This can lead to loss of optical performance, which is undesirable.

[0024] Typically, the gas flow from a gas lock funnel is directed symmetrically inwards towards an optical axis of the lithographic apparatus such that the net gas flow is directed generally along an optical axis of the lithographic apparatus (both into the projection system and out of the projection system towards the substrate table). That is, typically, the nozzle is arranged to direct gas preferentially towards a central portion of the gas lock funnel.

[0025] In the lithographic apparatus according to the second aspect the nozzle of the gas lock funnel is arranged to direct gas preferentially away from optical elements of the projection system. For example, the nozzle of the gas lock funnel may be arranged to direct gas preferentially towards a peripheral portion of the gas lock funnel or, more generally, the gas flow is directed preferentially towards a peripheral portion of the housing of the lithographic apparatus. Advantageously, this can ensure that in the event of a membrane rupturing, the amount of debris that is transferred to the optical elements of the projection system is reduced. This can avoid loss of optical performance, which is advantageous. Although in the construction of the nozzle according to the second aspect of the invention a small part of the gas flow may still be directed relatively centred, by directing gas preferentially towards a peripheral portion of the gas lock funnel is meant that at least a substantial part of the gas flow is skewed towards a peripheral portion of the housing of the lithographic apparatus and of the gas lock funnel, and thereby is skewed away from the EUV mirrors in the projection system that could receive debris.

[0026] Another consequence of the lithographic apparatus according to the second aspect, wherein the nozzle of the gas lock funnel is arranged to direct gas preferentially towards a peripheral portion of the gas lock funnel or towards walls of the projection system (in contrast to known arrangements) is that this will potentially alter the thermal load on the optical elements of the projection system from the gas from the nozzle. In turn, this can influence a deformation of these optical elements caused by the gas and therefore can influence one or more optical aberrations of the projection system. Any such aberrations are preferably corrected for by appropriate control over the projection optics. It will beappreciated that the corrections applied will be different for the lithographic apparatus according to the second aspect (wherein the nozzle of the gas lock funnel is arranged to direct gas preferentially towards a peripheral portion of the gas lock funnel or towards a wall of the projection system) and known arrangements (wherein the nozzle of the gas lock funnel is arranged to direct gas preferentially towards a central portion of the gas lock funnel).

[0027] In some embodiments of the lithographic apparatus, there may be no membrane supported by the gas lock funnel and provided in the optical path of the radiation. Such an arrangement may be referred to as an open configuration. It may be desirable to have the option to be able to choose to use a lithographic apparatus in either an open or a closed configuration. There is some advantage to the lithographic apparatus according to the second aspect, wherein the nozzle of the gas lock funnel is arranged to direct gas preferentially away from optical elements of the projection system, such that the heat load from the gas (and therefore the optical aberration treatment) is similar whether a membrane is present or not.

[0028] In summary, the lithographic apparatus according to the second aspect is advantageous as it allows for the option of providing a membrane supported by the gas lock funnel (i.e. allows the lithographic apparatus to be operated in a closed configuration) in such a way that in the event of the membrane rupturing there is little or no loss of optical performance. Equally, the lithographic apparatus according to the second aspect also allows for the lithographic apparatus to be operated in an open configuration without requiring a very different treatment of optical aberrations.

[0029] It will be appreciated that a gas being directed preferentially towards a peripheral portion of the gas lock funnel may mean the gas is not directed along an optical axis of the lithographic apparatus but towards (or along) one or more walls of the gas lock funnel or towards the housing of the lithographic apparatus.

[0030] The gas lock funnel may define a tapering volume through which the (converging radiation beam) propagates to reach the substrate. A dimension of the tapering volume may be smaller at an end proximate the substrate table and larger at an end proximate the projection optics. The tapering volume may be of the shape of a generalized, right frusto-conical volume, i.e. a right cone (i.e. the apex is above the center of the base) that has been truncated (i.e. the apex portion has been removed) and having a base that may be not circular or may be circular. For example, the base of the volume may have a shape that is similar to that of the target portion of the substrate but with rounded edges. The base of the volume may be generally kidney- or banana-shaped. Alternatively, the base of the volume may be generally circular. It will be appreciated that a gas being directed preferentially towards a peripheral portion of the gas lock funnel may mean the gas is not directed along an axis of the gas lock funnel (which may be generally aligned with an optical axis of the lithographic apparatus) but towards one or more walls of the gas lock funnel (and by extension, towards one or more walls of the housing of the projection system in the lithographic apparatus).

[0031] As used herein, an axis of a gas lock funnel may be defined as follows.

[0032] For embodiments wherein the gas lock funnel define a tapering volume of the shape of a generalized, right frusto-conical volume, the axis of the gas lock funnel may be the axis of the generalized, right frusto-conical volume.

[0033] Additionally or alternatively, the axis of the gas lock funnel may be defined as a direction such that in cross section in any plane perpendicular to the axis the shape of the interior of the gas lock funnel is generally the same. For example, the cross-sectional shape of the gas lock funnel in two cross sections that are perpendicular to the axis but at different axial positions will be similar shapes, i.e. one will be a scaled copy of the other. As noted above, the shapes may both be kidney-shaped or bananashaped.

[0034] Additionally or alternatively, the axis of the gas lock funnel may be defined as an optical axis of the lithographic apparatus within the gas lock funnel.

[0035] According to a third aspect of the present disclosure there is provided a lithographic apparatus comprising: a substrate table configured to hold a substrate; a projection system comprising optics configured to project radiation onto a target portion of the substrate; and a gas lock funnel arranged between the projection system and the substrate table; wherein a final optical element of the projection system has a central aperture; and wherein the gas lock funnel defines a nozzle that is arranged to provide a gas to an interior of the gas lock funnel such that on the projection system side of the gas lock funnel the gas is directed preferentially away from the central aperture in the final optical element.

[0036] The projection system may alternatively be referred to as an optical system. In the following optical system may be synonymous with projection system.

[0037] In some embodiments, the lithographic apparatus or lithographic system may comprise a membrane supported by the gas lock funnel and arranged between the projection system or optical system and the substrate table. Such an arrangement may be referred to as a closed configuration. The membrane is provided in the optical path of the radiation. The membrane may act to filter infrared (IR) and deep ultraviolet (DUV) radiation while transmitting EUV radiation. The nozzle may comprise two portions: a first portion arranged to deliver gas to the projection system or optical system side of the membrane and a second portion arranged to deliver gas to the substrate table side of the membrane. The gas flow from the first portion of the nozzle acts as a particle stopping flow to protect the membrane from particles within the projection optics housing impinging on the membrane.

[0038] In use, the gas lock funnel forms part of a dynamic gas lock. The gas lock funnel may be considered to define an exit aperture or exit window of a housing of the projection optics or optical system to allow the radiation to exit the housing of the projection optics or the optical system and be projected onto the target portion of the substrate. The membrane acts to suppress out of band radiation (i.e. radiation that is not being used for lithographic exposure) from being transmitted to the substrate and to protect the projection system or optical system optics from resist outgassing.

[0039] In use, the membrane can rupture, which can cause debris from the membrane. This can be transported into the optical elements in the projection system or optical system by the gas flow from the nozzle. This can lead to loss of optical performance, which is undesirable.

[0040] There is a move for newer EUV lithographic systems to have an increased numerical aperture (NA) in order to image smaller features. In some such high-NA systems, the last optical element in the projection system or optical system is a large concave mirror, which is provided with a central aperture. The radiation is directed through this aperture by an optical element that is close to a field plane of the lithographic apparatus. The radiation propagates through the aperture towards a penultimate optical element proximate to the gas lock funnel, which directs the radiation to the last optical element, which focuses the radiation through the gas lock funnel to the target portion of the substrate.

[0041] Typically, the gas flow from a gas lock funnel is directed symmetrically inwards towards an optical axis of the lithographic apparatus or lithographic system such that the net gas flow is directed generally along an optical axis of the lithographic apparatus or lithographic system (both into the projection system / optical system and out of the projection system / optical system towards the substrate table). However, in high-NA systems such an arrangement directs the gas flow towards the central aperture in the last optical element, i.e. towards an optical element that is in or close to a field plane of the lithographic apparatus or lithographic system. Any contamination on such optical elements is especially disadvantageous as it can negatively affect slit uniformity (i.e. a uniformity in the dose of radiation delivered to different parts of the target portion of the substrate), which can in turn adversely affect the critical dimension uniformity (CDU).

[0042] The nozzle of the lithographic apparatus according to the third aspect is arranged to provide a gas that is directed preferentially away from any optical elements in or close to a field plane of the lithographic apparatus. This is advantageous as this can avoid loss of slit uniformity and the loss of critical dimension uniformity in the event of a membrane rupturing.

[0043] The final optical element of the projection system (or optical system) may be an optical element of the projection system (or optical system) that radiation is incident on before being projected onto the target portion of the substrate.

[0044] Another consequence of the lithographic apparatus according to the third aspect, wherein the nozzle of the gas lock funnel is arranged to direct gas preferentially away from the central aperture in the final optical element (in contrast to known arrangements) is that this will potentially alter the thermal load on the optical elements of the projection system (or optical system) from the gas from the nozzle. In turn, this can influence a deformation of these optical elements caused by the gas and therefore can influence one or more optical aberrations of the projection system (or optical system). Any such aberrations are preferably corrected for by appropriate control over the projection optics. It will be appreciated that the corrections applied will be different for the lithographic apparatus according to the third aspect (wherein the nozzle of the gas lock funnel is arranged to direct gas preferentiallyaway from the central aperture in the final optical element) and known arrangements (wherein the nozzle of the gas lock funnel is arranged to direct gas preferentially towards a central portion of the gas lock funnel).

[0045] In some embodiments of the lithographic apparatus or lithographic system there may be no membrane supported by the gas lock funnel and provided in the optical path of the radiation. Such an arrangement may be referred to as an open configuration. It may be desirable to have the option to be able to choose to use a lithographic apparatus or lithographic system in either an open or a closed configuration. There is some advantage to the lithographic apparatus according to the third aspect, wherein the nozzle of the gas lock funnel is arranged to direct gas preferentially away from the central aperture in the final optical element, such that the heat load from the gas (and therefore the optical aberration treatment) is similar whether a membrane is present or not.

[0046] In summary, the lithographic apparatus according to the third aspect is advantageous as it allows for the option of providing a membrane supported by the gas lock funnel (i.e. allows the lithographic apparatus to be operated in a closed configuration) in such a way that in the event of the membrane rupturing there is little or no no loss of optical performance. Equally, the lithographic apparatus according to the third aspect also allows for the lithographic apparatus to be operated in an open configuration without requiring a very different treatment of optical aberrations.

[0047] The nozzle of the lithographic apparatus according to the third aspect may be arranged to provide the gas to the interior of the gas lock funnel such that on the projection system side of the gas lock funnel the gas is directed preferentially away from optical elements of the projection system.

[0048] The projection system of the lithographic apparatus according to the third aspect may comprise a second optical element arranged such that a line extending from the gas lock funnel to the central aperture of the final optical element intersects the second optical element.

[0049] Advantageously, because the nozzle is arranged to direct the gas preferentially away from the central aperture in the final optical element, the gas is directed away from the second optical element, which may ensure that remnants of the membrane are directed away from the second optical element in the event that the membrane ruptures.

[0050] The projection system of the lithographic apparatus according to the third aspect may comprise a penultimate optical element. The second optical element may direct the radiation to the penultimate optical element and the penultimate optical element may direct the radiation to the final optical element.

[0051] The second optical element may, for example, be disposed in or close to a field plane of the lithographic apparatus.

[0052] In use, the lithographic apparatus is operable to illuminate a reticle (a patterning device) with a radiation beam and to use the projection system to form an image of the reticle on a substrate (for example a resist-coated silicon wafer) supported by the substrate table. That is, a plane that the reticle is disposed in is optically conjugate to a plane that the substrate is disposed in. The reticle andsubstrate planes may be referred to as field planes. As used herein, a field plane is intended to mean any plane that is optically conjugate to the reticle and substrate planes.

[0053] The second optical element may intersect a field plane of the lithographic apparatus.

[0054] For such embodiments it is particularly important that a portion of the second optical element that intersects the field plane remains free from contamination. This portion may be referred to as a caustic region of the optical element.

[0055] The lithographic system or apparatus of the first, second or third aspects may further comprise a membrane supported by the gas lock funnel and arranged between: the projection system or the optical system; and the substrate table or the second system. The nozzle of the gas lock funnel may be arranged to provide the gas in the vicinity of the membrane.

[0056] The nozzle may be arranged to provide the gas such that on the projection system or optical system side of the gas lock funnel the gas is directed away from any optical elements in or close to a field plane of the lithographic apparatus. That is, the nozzle may be arranged to provide the gas such that none, or a negligible amount, of the gas impinges on any optical elements in or close to a field plane of the lithographic apparatus.

[0057] Any contamination on such optical elements that are in or close to a field plane is especially disadvantageous as it can negatively affect slit uniformity (i.e. a uniformity in the dose of radiation delivered to different parts of the target portion of the substrate), which can in turn adversely affect the critical dimension uniformity (CDU). An arrangement wherein the nozzle is arranged to provide a gas that is directed preferentially away from any optical elements in or close to a field plane of the lithographic apparatus is advantageous as it can avoid such loss or slit uniformity and the loss of critical dimension uniformity.

[0058] The nozzle may be arranged to provide the gas to the interior of the gas lock funnel such that on the projection system or optical system side of the gas lock funnel the gas is directed asymmetrically inwards towards an optical axis of the lithographic apparatus or lithographic system such that the net gas flow is directed at a non-zero angle to the optical axis.

[0059] The nozzle may be arranged such that the net gas flow is skewed away from the optical axis (at a non-zero angle threreto) in any direction. That is, the nozzle may be arranged such that the asymmetry of the flow inwards towards the optical axis may be in any direction. It will be appreciated that, in general, one particular direction may be favored or preferable, depending on the design of the optics of the projection system or optical system.

[0060] As described above, an internal volume of the gas lock funnel may be of the shape of a generalized, right frusto-conical volume with a base having a shape that is similar to that of the target portion of the substrate. The shape of the target portion of the substrate may be elongate (and may be curved) having a longer dimension and a shorter dimension. The asymmetry of the flow inwards towards the optical axis may be in the direction of the shorter dimension.

[0061] The target portion of the substrate may be elongate, having a longer dimension and a shorter dimension, and the nozzle may be arranged such that the asymmetry of the flow inwards towards the optical axis is in the direction of the shorter dimension.

[0062] The lithographic apparatus or lithographic system may be a scanning lithographic apparatus or lithographic system and the nozzle may be arranged such that the asymmetry of the flow inwards towards the optical axis is in a scanning direction of the lithographic apparatus.

[0063] The nozzle may be arranged such that in a first linear direction in a plane perpendicular to an optical axis of the lithographic apparatus or lithographic system flow is provided in one direction only.

[0064] The first linear dimension may be referred to as a y-direction. That is, the nozzle is arranged to provide flow is in one direction only (for example only one of the positive y-direction or the negative y-direction). The first linear dimension may be aligned with a shorter dimension of the target portion of the substrate. The first linear dimension may be aligned with a scanning direction of lithographic apparatus.

[0065] Alternatively, in some embodiments the nozzle may be arranged such that in the first linear direction in a plane perpendicular to an optical axis of the lithographic apparatus or lithographic system flow is provided in two opposite directions and wherein the flow rates in the two opposite directions are not equal.

[0066] The nozzle may be arranged such that in a second linear direction in a plane perpendicular to the optical axis of the lithographic apparatus or lithographic system flow is provided in two opposing directions.

[0067] Such an arrangement may provide a control knob that can be used to optimize the dynamic gas lock. For example, this may be helpful in achieving a more optimal spatial distribution of the pressure difference across the membrane (when present) and / or in case smaller gas expansion and thus lower thermal impact is preferred. In general, the combination of different segmented flow inlets above and below membrane can be used as a control knob to optimize a spatial distribution of the pressure difference across the membrane (when present).

[0068] The second linear dimension may be generally perpendicular to the first linear dimension. The second linear dimension may be referred to as an x-direction. The second linear dimension may be aligned with a longer dimension of the target portion of the substrate. The second linear dimension may be aligned with a non-scanning direction of lithographic apparatus.

[0069] Alternatively, in some embodiments the nozzle may be arranged such that substantially no flow is provided in the second linear direction in a plane perpendicular to the optical axis of the lithographic apparatus.

[0070] The nozzle may be arranged to provide the gas to the interior of the gas lock funnel such that on the projection system or optical system side of the gas lock funnel the gas is directed so as to swirl about an optical axis of the lithographic apparatus or lithographic system.

[0071] The nozzle may comprise one or more apertures shaped so as to impart swirl to the gas on the projection system or optical system side of the membrane.

[0072] The apertures may, for example, impart the swirl to the gas according to the Coanda effect.

[0073] The nozzle may comprise one or more guide vanes shaped so as to impart swirl to the gas on the projection system or optical system side of the membrane.

[0074] The guide vanes may, for example, extend across an exit opening or slit. The guide vanes may act as aerofoils.

[0075] The nozzle may comprise an opening on a wall of the gas lock funnel, wherein said opening extends around only a portion of a perimeter of the gas lock funnel.

[0076] The opening may be a slit that extends around a portion of an axis of the gas lock funnel. In general, the nozzle may comprise a plurality of openings on the wall of the gas lock funnel. The plurality of openings may extend around only a portion of a perimeter of the gas lock funnel. The sizes, positions and spacings of the plurality of openings may be such that a desired net gas flow is achieved on the projection system side of the gas lock funnel.

[0077] The gas lock funnel may comprise a seal shield having inlets and / or nozzles arranged to provide a gas to the opening.

[0078] The lithographic apparatus or lithographic system may further comprise a debris catcher arranged to catch debris. For example, such debris may comprise membrane fragments in the event of rupture of a membrane. Additionally or alternatively, the debris may comprise fuel debris, for example tin debris, from a radiation source.

[0079] The lithographic apparatus or lithographic system may further comprise a gas source operable to deliver gas to the nozzle.

[0080] The gas may, for example, comprise hydrogen gas. Additionally or alternatively, the gas may comprise nitrogen and / or clean air (for example extreme clean dry air or XCDA).

[0081] The projection system or optical system may comprise a housing in which the optics are disposed. The housing may define an aperture which, in use, the radiation propagates through. The nozzle may be arranged to provide the gas to the interior of the gas lock funnel such that on the projection system or optical system side of the gas lock funnel a net direction of the gas does not point towards the aperture.

[0082] It will be appreciated that as used here, the net direction of the gas not pointing towards the aperture is intended to mean that if a line or arrow pointing in the direction of the net direction of the gas was extended towards the projection system or optical system it would not intersect with or pass through the aperture. Rather, it would intersect with a wall of the housing.

[0083] In order to facilitate such embodiments, a shape of the gas lock funnel may be altered relative to known arrangements. For example, the gas lock funnel may define a volume, a shape of which is generally of the form of a right frusto-conical volume, which defines the axis of thegas lock funnel. In order to ensure that a net direction of the gas flow on the optical system side does not point towards the aperture, a divergence or tapering of the gas lock funnel may be increased.

[0084] The nozzle may comprise a plurality of apertures defined in a wall of the gas lock funnel, the plurality of apertures may be circumferentially spaced around an axis of the gas lock funnel.

[0085] For example, there may be of the order of 12 or 13 apertures circumferentially distributed around the axis of the gas lock funnel.

[0086] In some embodiments, the nozzle may comprise more than one (for example two or three) rows of apertures defined in a wall of the gas lock funnel and circumferentially spaced around an axis of the gas lock funnel.

[0087] The plurality of apertures defined in the wall of the gas lock funnel may be asymmetrically arranged around the axis of the gas lock funnel.

[0088] For example, by providing more apertures on a first circumferential portion than a second circumferential portion, a net direction of the gas flow may be tilted away from an axis of the gas lock funnel towards the second circumferential portion. In order to retrofit such an arrangement to existing gas lock funnels, one or more existing apertures on the second circumferential portion may be blocked.

[0089] The plurality of apertures defined in the wall of the gas lock funnel may comprise at least two sets of apertures, each set of apertures comprising at least one of the plurality of apertures. Each set of apertures may be provided on a different circumferential portion of the gas lock funnel. The apertures of any given set may share a common inlet or manifold and each set may have a different manifold or inlet.

[0090] This may provide a more controllable and versatile arrangement for controlling a net direction of the gas flow. For example, by independently controlling a flow rate provided to the manifolds of different sets of apertures, a net direction of the gas flow may be controlled.

[0091] According to a fourth aspect of the present disclosure there is provided an assembly for use in the lithographic apparatus or lithographic system of the first, second or third aspects of the present disclosure, the assembly comprising the gas lock funnel. Optionally, the assembly may further comprise a membrane supported by the gas lock funnel.

[0092] According to a fifth aspect of the present disclosure there is provided an assembly for use in a lithographic apparatus or lithographic system, the assembly comprising a gas lock funnel; wherein the gas lock funnel defines a nozzle that is arranged to provide a gas in to an interior of the gas lock funnel such that on a first side of the gas lock funnel the gas is directed preferentially towards a peripheral portion of the gas lock funnel.

[0093] The first side of the gas lock funnel may be the side which faces a larger portion of the tapering gas lock funnel.

[0094] The assembly may further comprise a membrane supported by the gas lock funnel.

[0095] The nozzle may be arranged such that the gas is not directed along an axis of the gas lock funnel.

[0096] The gas lock funnel may define a volume, a shape of which is generally of the form of a generalized, right frusto-conical volume, which defines the axis of the gas lock funnel.

[0097] The nozzle may be arranged to provide the gas to the interior of the gas lock funnel such that on the first side of the gas lock funnel the gas is directed asymmetrically inwards towards an axis of the gas lock funnel such that the net gas flow is directed at a non-zero angle to the axis.

[0098] The nozzle may be arranged such that the net gas flow is skewed away from the axis (at a non-zero angle threreto) in any direction. That is, the nozzle may be arranged such that the asymmetry of the flow inwards towards the axis may be in any direction. It will be appreciated that, in general, one particular direction may be favored or preferable, depending on the design of the optics of the projection system or optical system of the lithographic apparatus or lithographic system that the assembly is used in.

[0099] A cross sectional shape of the gas lock funnel in a plane perpendicular to its axis may be elongate, having a longer dimension and a shorter dimension, and the nozzle may be arranged such that the asymmetry of the flow inwards towards the axis is in the direction of the shorter dimension.[000100] The nozzle may be arranged such that in a first linear direction in a plane perpendicular to an axis of the gas lock funnel the flow is provided in one direction only.[000101] The first linear dimension may be referred to as a y-direction. That is, the nozzle is arranged to provide flow is in one direction only (for example only one of the positive y-direction or the negative y-direction). The first linear dimension may be aligned with a shorter dimension of the cross sectional shape of the gas lock funnel in a plane perpendicular to its axis. In use, the first linear dimension may be aligned with a scanning direction of a lithographic apparatus.[000102] Alternatively, in some embodiments the nozzle may be arranged such that in the first linear direction in a plane perpendicular to the axis of the gas lock funnel flow is provided in two opposite directions and wherein the flow rates in the two opposite directions are not equal.[000103] The nozzle may be arranged such that in a second linear direction in a plane perpendicular to the axis of the gas lock funnel gas flow is provided in two opposing directions.[000104] Such an arrangement may provide a control knob that can be used to optimize the dynamic gas lock. For example, this may be helpful in achieving a more optimal spatial distribution of the pressure difference across a membrane (when present) and / or in case smaller gas expansion and thus lower thermal impact is preferred. In general, the combination of different segmented flow inlets above and below membrane can be used as a control knob to optimize a spatial distribution of the pressure difference across the membrane (when present).[000105] The second linear dimension may be generally perpendicular to the first linear dimension. The second linear dimension may be referred to as an x-direction. The second linear dimension may be aligned with a longer dimension of the cross sectional shape of the gas lock funnel in a plane perpendicular to its axis. In use, the second linear dimension may be aligned with a non-scanning direction of lithographic apparatus.[000106] Alternatively, in some embodiments the nozzle may be arranged such that substantially no flow is provided in the second linear direction in a plane perpendicular to the axis of the gas lock funnel. [000107] The nozzle may be arranged to provide the gas to the interior of the gas lock funnel such that on the first side of the gas lock funnel the gas is directed so as to swirl about an axis of the gas lock funnel.[000108] The nozzle may comprise one or more apertures shaped so as to impart swirl to the gas on the first side of the membrane.[000109] The apertures may, for example, impart the swirl to the gas according to the Coanda effect. [000110] The nozzle may comprise one or more guide vanes shaped so as to impart swirl to the gas on the first side of the membrane.[000111] The guide vanes may, for example, extend across an exit opening or slit. The guide vanes may act as aerofoils.[000112] The nozzle may comprise an opening on a wall of the gas lock funnel, wherein said opening extends around only a portion of a perimeter of the gas lock funnel.[000113] The opening may be a slit that extends around a portion of an axis of the gas lock funnel. [000114] The nozzle may comprise a plurality of apertures defined in a wall of the gas lock funnel. The plurality of apertures may be circumferentially spaced around an axis of the gas lock funnel.[000115] For example, there may be of the order of 12 or 13 apertures circumferentially distributed around the axis of the gas lock funnel.[000116] In some embodiments, the nozzle may comprises more than one (for example two or three) rows of apertures defined in a wall of the gas lock funnel and circumferentially spaced around an axis of the gas lock funnel.[000117] The plurality of apertures defined in the wall of the gas lock funnel may be asymmetrically arranged around the axis of the gas lock funnel.[000118] For example, by providing more apertures on a first circumferential portion than a second circumferential portion, a net direction of the gas flow may be tilted away from an axis of the gas lock funnel towards the second circumferential portion. In order to retrofit such an arrangement to existing gas lock funnels, one or more existing apertures on the second circumferential portion may be blocked. [000119] The plurality of apertures defined in the wall of the gas lock funnel may comprise at least two sets of apertures. Each set of apertures may comprise at least one of the plurality of apertures. Each set of apertures may be provided on a different circumferential portion of the gas lock funnel. The apertures of any given set may share a common inlet or manifold and each set may have a different manifold or inlet.[000120] This may provide a more controllable and versatile arrangement for controlling a net direction of the gas flow. For example, by independently controlling a flow rate provided to the manifolds of different sets of apertures, a net direction of the gas flow may be controlled.[000121] According to a sixth aspect of the present disclosure there is provided a lithographic apparatus comprising: a substrate table configured to hold a substrate; a projection system comprising optics configured to project radiation onto a target portion of the substrate; and a gas lock funnel arranged between the projection system and the substrate table; wherein the gas lock funnel defines a nozzle that is arranged to provide a gas to an interior of the gas lock funnel such that on the projection system side of the gas lock funnel the gas is directed preferentially towards a peripheral portion of the gas lock funnel.[000122] The lithographic apparatus according to the sixth aspect of the present disclosure may incorporate any features of the first, second and third aspects of the present disclosure.BRIEF DESCRIPTION OF THE DRAWINGS[000123] Embodiments will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:Figure 1 depicts a lithographic system comprising a lithographic apparatus and a radiation source;Figure 2A shows a first example gas flow architecture for a dynamic gas lock that may form part of the lithographic apparatus shown in Figure 1, which may be referred to as an open configuration;Figure 2B shows a second example gas flow architecture for a dynamic gas lock that may form part of the lithographic apparatus shown in Figure 1, which may be referred to as a closed configuration;Figure 3 is a schematic illustration of an open configuration, which comprises a flow distribution chamber defined in a wall of the gas lock funnel and which surrounds the optical axis of the lithographic apparatus;Figure 4A is a schematic cross section illustration of a closed arrangement (shown in a first plane), which comprises a flow distribution chamber defined in a wall of the gas lock funnel and which surrounds the optical axis of the lithographic apparatus;Figure 4B is a schematic cross section illustration of the closed arrangement shown in Figure 4A (shown in a second plane that is parallel to the first plane), which comprises a flow distribution chamber defined in a wall of the gas lock funnel and which surrounds the optical axis of the lithographic apparatus;Figure 4C is a schematic cross section illustration of the closed arrangement shown in Figures 4A and 4B (shown in a third plane that is orthogonal to the first and second planes), which comprises a flow distribution chamber defined in a wall of the gas lock funnel and which surrounds the optical axis of the lithographic apparatus;Figure 5 schematically shows an arrangement of the last three optical elements of the projection system of a lithographic apparatus of the type shown in Figure 1;Figure 6A is a first modified version of the schematic cross section illustration of a closed arrangement as shown in Figures 4A to 4C (shown in the same plane as Figure 4A), which comprises aflow distribution chamber defined in a wall of the gas lock funnel and which surrounds the optical axis of the lithographic apparatus;Figure 6B is a second modified version of the schematic cross section illustration of a closed arrangement as shown in Figures 4A to 4C (shown in the same plane as Figure 4A), which comprises a flow distribution chamber defined in a wall of the gas lock funnel and which surrounds the optical axis of the lithographic apparatus;Figure 7 is a schematic illustration of a closed arrangement as shown in Figures 4A to 4C wherein the nozzle is arranged to provide the gas in the vicinity of the membrane such that towards the projection system (for example on the projection system PS side of the membrane when present) the gas is directed so as to swirl about an optical axis of the lithographic apparatus;Figure 8A schematically shows a known type of a dynamic gas lock in the vicinity of the opening in an enclosing structure of the radiation source shown in Figure 1 ;Figure 8B shows an enlarged portion of the known arrangement shown in Figure 8A;Figure 9A shows an enlarged portion of a first embodiment of a new arrangement for a dynamic gas lock in the vicinity of the opening in an enclosing structure of the radiation source shown in Figure 1 according to the present disclosure; andFigure 9B shows an enlarged portion of a second embodiment of a new arrangement for a dynamic gas lock in the vicinity of the opening in an enclosing structure of the radiation source shown in Figure 1 according to the present disclosure.DETAILED DESCRIPTION[000124] Figure 1 shows a lithographic system comprising a radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate an EUV radiation beam B and to supply the EUV radiation beam B to the lithographic apparatus LA. The lithographic apparatus LA comprises an illumination system IL, a support structure MT configured to support a patterning device MA (e.g., a mask), a projection system PS and a substrate table WT configured to support a substrate W.[000125] The illumination system IL is configured to condition the EUV radiation beam B before the EUV radiation beam B is incident upon the patterning device MA. Thereto, the illumination system IL may include a facetted field mirror device 10 and a facetted pupil mirror device 11. The faceted field mirror device 10 and faceted pupil mirror device 11 together provide the EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. The illumination system IL may include other mirrors or devices in addition to, or instead of, the faceted field mirror device 10 and faceted pupil mirror device 11.[000126] After being thus conditioned, the EUV radiation beam B interacts with the patterning device MA. As a result of this interaction, a patterned EUV radiation beam B’ is generated. The projection system PS is configured to project the patterned EUV radiation beam B’ onto the substrateW. For that purpose, the projection system PS may comprise a plurality of mirrors 13, 14 which are configured to project the patterned EUV radiation beam B’ onto the substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the patterned EUV radiation beam B’, thus forming an image with features that are smaller than corresponding features on the patterning device MA. For example, a reduction factor of 4 or 8 may be applied. Although the projection system PS is illustrated as having only two mirrors 13, 14 in Figure 1, the projection system PS may include a different number of mirrors (e.g., six or eight mirrors).[000127] The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus LA aligns the image, formed by the patterned EUV radiation beam B’, with a pattern previously formed on the substrate W.[000128] A relative vacuum, i.e. a small amount of gas (e.g. hydrogen) at a pressure well below atmospheric pressure, may be provided in the radiation source SO, in the illumination system IL, and / or in the projection system PS.[000129] The radiation source SO shown in Figure 1 is, for example, of a type which may be referred to as a laser produced plasma (LPP) source. A laser system 1, which may, for example, include a CO2 laser, is arranged to deposit energy via a laser beam 2 into a fuel, such as tin (Sn) which is provided from, e.g., a fuel emitter 3. Although tin is referred to in the following description, any suitable fuel may be used. The fuel may, for example, be in liquid form, and may, for example, be a metal or alloy. The fuel emitter 3 may comprise a nozzle configured to direct tin, e.g. in the form of droplets, along a trajectory towards a plasma formation region 4. The laser beam 2 is incident upon the tin at the plasma formation region 4. The deposition of laser energy into the tin creates a tin plasma 7 at the plasma formation region 4. Radiation, including EUV radiation, is emitted from the plasma 7 during deexcitation and recombination of electrons with ions of the plasma.[000130] The EUV radiation from the plasma is collected and focused by a collector 5. Collector 5 comprises, for example, a near-normal incidence radiation collector 5 (sometimes referred to more generally as a normal-incidence radiation collector). The collector 5 may have a multilayer mirror structure which is arranged to reflect EUV radiation (e.g., EUV radiation having a desired wavelength such as 13.5 nm). The collector 5 may have an ellipsoidal configuration, having two focal points. A first one of the focal points may be at the plasma formation region 4, and a second one of the focal points may be at an intermediate focus 6, as discussed below.[000131] The laser system 1 may be spatially separated from the radiation source SO. Where this is the case, the laser beam 2 may be passed from the laser system 1 to the radiation source SO with the aid of a beam delivery system (not shown) comprising, for example, suitable directing mirrors and / or a beam expander, and / or other optics. The laser system 1, the radiation source SO and the beam delivery system may together be considered to be a radiation system.[000132] Radiation that is reflected by the collector 5 forms the EUV radiation beam B. The EUV radiation beam B is focused at intermediate focus 6 to form an image at the intermediate focus 6 of theplasma present at the plasma formation region 4. The image at the intermediate focus 6 acts as a virtual radiation source for the illumination system IL. The radiation source SO is arranged such that the intermediate focus 6 is located at or near to an opening 8 in an enclosing structure 9 of the radiation source SO.[000133] Although Figure 1 depicts the radiation source SO as a laser produced plasma (LPP) source, any suitable source such as a discharge produced plasma (DPP) source or a free electron laser (FEL) may be used to generate EUV radiation.[000134] Embodiments of the present disclosure relate to new arrangements of dynamic gas lock in a lithographic system of the type shown in Figure 1. Such dynamic gas locks may be provided in various parts of the lithographic system.[000135] For example, the lithographic system shown in Figure 1 may comprise a dynamic gas lock in the vicinity of the opening 8 in an enclosing structure 9 of the radiation source SO, where the radiation beam B exits the radiation source and enters the illumination system IL. Embodiments of the present disclosure relating to such a dynamic gas lock are described below with reference to Figures 8 to 9B.[000136] Additionally or alternatively, the lithographic apparatus LA shown in Figure 1 may comprise a dynamic gas lock in the vicinity of an exit aperture or window of the projection system PS, where the patterned radiation beam B’ leaves the projection system PS and propagates to the substrate W. Embodiments of the present disclosure relating to such a dynamic gas lock are now described with reference to Figures 2A to 7.[000137] Figures 2A and 2B show two example gas flow architectures for a dynamic gas lock 100 that may form part of the lithographic apparatus LS shown in Figure 1. The dynamic gas 100 locks each comprise a gas lock funnel 102.[000138] The gas lock funnel 102 may be considered to define an exit aperture (see Figure 2 A) or exit window (see Figure 2B) of a housing of the projection system PS to allow the radiation B’ to exit the housing of the projection system PS and be projected onto a target portion of the substrate W.[000139] Figure 2 A shows an arrangement with a gas flow that is directed inwards towards an optical axis 104 of the lithographic apparatus LA. The gas flows inwards towards the optical axis 104 from all directions around the axis and forms: (i) an up flow 106a that flows into the projection optics housing; and (ii) a down flow 106b that flows down to the substrate W. Such an arrangement may be referred to as an open configuration. During exposure, the down flow 106b acts to suppress contaminants, in particular resist outgassing, from migrating into the housing of the projection system PS (where such contaminants can damage optical elements, degrading the optical performance of the projection optics). [000140] Figure 2B shows an arrangement wherein a thin membrane 108 is provided in the optical path to filter infrared (IR) and deep ultraviolet (DUV) radiation, while transmitting EUV radiation. Such an arrangement may be referred to as a closed configuration. The arrangement shown in Figure 2B is provided with two gas flows in towards an optical axis 104 of the lithographic apparatus: an up flow 106a and a down flow 106b. For the arrangement shown in Figure 2B, the up flow 106a acts as aparticle stopping flow to protect the membrane 108 from particles within the housing of the projection system PS impinging on the membrane 108 and can also be used to minimize a pressure difference across the membrane 108.[000141] One arrangement (shown schematically in Figure 3 for an open configuration) comprises a flow distribution chamber 110, which is defined in a wall of the gas lock funnel 102 and which surrounds the optical axis 104 of the lithographic apparatus LA. The flow distribution chamber 110 therefore provides a generally ring-shaped volume that surrounds the optical axis 104 of the lithographic apparatus. The flow distribution chamber is in fluid connection with an interior of the housing of the projection system PS from all sides around the optical axis 104. A flow of gas is provided into the flow distribution chamber via a gas inlet 112. In order to provide a uniform flow into the interior of the gas lock funnel 102 (rather than, for example, a greater flow from parts of the flow distribution chamber that are closer to the gas inlet 112), a restrictive outlet nozzle 114 is provided between the flow distribution chamber 112 and the interior of the gas lock funnel 102 substantially surrounding the optical axis 104.[000142] A closed arrangement is shown schematically in Figures 4A, 4B and 4C. Figures 4A and 4B are cross sections in two first and second planes, which are mutually parallel and Figure 4C is a cross section in a third plane that is orthogonal to the first and second planes. The closed arrangement comprises a flow distribution chamber 110, which is defined in a wall of the gas lock funnel 102 and which surrounds the optical axis 104 of the lithographic apparatus LA. The flow distribution chamber 110 therefore provides a generally ring-shaped volume that surrounds the optical axis 104 of the lithographic apparatus. The flow distribution chamber 110 is in fluid connection with an interior of the housing of the projection system PS via a seal shield 116. A flow of gas is provided into the flow distribution chamber 110 via a gas inlet 112 (see Figure 4B).[000143] As can be seen in Figure 4 A, the flow distribution chamber 110 is in fluid connection with first pre-expansion chambers 118, which are in fluid connection with a first nozzle 114a. It will be appreciated from Figures 4 A, 4B and 4C that the first pre-expansion chambers 118 do not extend around the entire perimeter of the axis 104. The first nozzle 114a is on the projection system PS side of the membrane 108.[000144] As can be seen in Figure 4B, the flow distribution chamber 110 is in fluid connection with second pre-expansion chambers 120, which are in fluid connection with a second nozzle 114b. It will be appreciated from Figures 4A, 4B and 4C that the second pre-expansion chambers 120 do not extend around the entire perimeter of the axis 104. The second nozzle 114b is on the substrate W side of the membrane 108.[000145] The membrane 108 acts to suppress out of band radiation (i.e. radiation that is not being used for lithographic exposure) from being transmitted to the substrate W and to protect the projection system PS optics from resist outgassing.[000146] The membrane is provided in the optical path of the radiation B’. The membrane may act to fdter infrared (IR) and deep ultraviolet (DUV) radiation while transmitting EUV radiation.[000147] The nozzle may comprise two portions: a first portion 114a arranged to deliver gas to the projection system PS side of the membrane 108 and a second portion 114b arranged to deliver gas to the substrate table WT side of the membrane 108. The gas flow from the first portion of the nozzle 114a acts as a particle stopping flow to protect the membrane 108 from particles within the projection optics PS housing impinging on the membrane 108.[000148] The present disclosure relates to both closed arrangements of the type shown on Figures 4 A, 4B and 4C and open arrangements of the type shown in Figure 3. However, the present disclosure may have particular application to closed arrangements of the type shown on Figures 4A, 4B and 4C. In particular, some embodiments of the present disclosure relate to arrangements wherein the gas lock funnel defines a nozzle 114a, 114b that is arranged to provide a gas that is directed towards the projection system PS such that the gas 106a is directed preferentially towards a peripheral portion of the gas lock funnel.[000149] In use, the membrane 108 can rupture, which can cause debris from the membrane 108. This can be transported into the optical elements in the projection system PS by the gas flow 106a from the nozzle. This can lead to loss of optical performance, which is undesirable.[000150] Typically, the gas flow 106a from a gas lock funnel is directed symmetrically inwards towards an optical axis 104 of the lithographic apparatus LA such that the net gas flow is directed generally along an optical axis 104 of the lithographic apparatus LA (both into the projection system PS and out of the projection system PS towards the substrate table WT). This is indicated schematically in Figures 2A to 4C by arrows 122. That is, in known arrangements, typically, the nozzle is arranged to direct gas preferentially towards a central portion of the gas lock funnel.[000151] In the lithographic apparatus LA according to some embodiments of the present disclosure, the nozzle 114a, 114b of the gas lock funnel 102 is arranged to direct gas preferentially towards a peripheral portion of the gas lock funnel 102 or towards a wall of the projection system PS. Advantageously, this can ensure that in the event of the membrane rupturing, the amount of debris that is transferred to the optical elements of the projection system PS is reduced. This can avoid loss of optical performance, which is advantageous. Such arrangements wherein the nozzle 114a, 114b of the gas lock funnel 102 is arranged to direct gas preferentially towards a peripheral portion of the gas lock funnel 102 or towards a wall of the projection system PS are shown in Figures 6A to 7 and discussed below.[000152] It will be appreciated that a gas 106a being directed preferentially towards a peripheral portion of the gas lock funnel 102 may mean the gas 106a is not directed along an optical axis 104 of the lithographic apparatus LA but towards one or more walls of the gas lock funnel 102 or towards one or more walls of the projection system PS.[000153] The gas lock funnel 102 may define a tapering volume through which the (converging radiation beam) propagates to reach the substrate W. A dimension of the tapering volume may be smaller at an end proximate the substrate table WT and larger at an end proximate the projection optics PS. The tapering volume may be of the shape of a generalized, right frusto-conical volume, i.e. a right cone (i.e. the apex is above the center of the base) that has been truncated (i.e. the apex portion has been removed) and having a base that is not circular. The base of the volume may have a shape that is similar to that of the target portion of the substrate W but with rounded edges. The base of the volume may be generally kidney -shaped. It will be appreciated that a gas 106a being directed preferentially towards a peripheral portion of the gas lock funnel 102 may mean the gas 106a is not directed along an axis of the gas lock funnel (which may be generally aligned with an optical axis 104 of the lithographic apparatus LA) but towards one or more walls of the gas lock funnel 102.[000154] Some embodiments of the present disclosure relate to a lithographic apparatus LA wherein a final optical element of the projection system PS has a central aperture; and wherein nozzle 114a, 114b that is arranged to provide a gas to an interior of the gas lock gunnel (for example in the vicinity of the membrane 108) such that on the projection system PS side of the membrane 108 the gas 106a is directed preferentially away from the central aperture in the final optical element.[000155] Figure 5 schematically shows an arrangement 200 of the last three optical elements of the projection system PS.[000156] There is a move for newer EUV lithographic systems to have an increased numerical aperture in order to image smaller features. In some such high-NA systems, the last optical element 202 in the projection system is a large concave mirror, which is provided with a central aperture 204. The radiation is directed through this aperture 204 by an optical element that is close to a field plane of the lithographic apparatus LA (for example an optical element in region 206). The radiation propagates through the aperture 204 towards a penultimate optical element 208 proximate to the gas lock funnel 102, which directs the radiation to the last optical element 202, which focuses the radiation through the gas lock funnel 102 to the target portion of the substrate W.[000157] Also shown in Figure 5 are walls 210, 212, 214 that define part of an interior of the projection system PS and the dynamic gas lock 100 is indicated schematically as a dashed box. Also shown in Figure 5 is a part of a wall 216 that defines an environment within which the substrate W is disposed (the radiation propagates through an aperture in the wall 216 that defines an environment within which the substrate W).[000158] The last optical element 202, the penultimate optical element 208 and the walls 212, 214 that define part of an interior of the projection system PS define an environment 218 that a portion of the gas above the membrane is directed into. It will be appreciated that the gas may be removed from the environment via one or more vacuum pumps (not shown).[000159] Typically, the gas flow from a gas lock funnel is directed symmetrically inwards towards an optical axis of the lithographic apparatus LA such that the net gas flow is directed generally alongan optical axis of the lithographic apparatus (both into the projection system PS and out of the projection system PS towards the substrate table). A direction of flow within the environment 218 above the membrane for such a typical arrangement is indicated by arrow 220. However, in high-NA systems such an arrangement directs the gas flow 106a towards the central aperture 204 in the last optical element 202, i.e. towards an optical element that is in or close to a field plane of the lithographic apparatus LA (in region 206). Any contamination on such optical elements is especially disadvantageous as it can negatively affect slit uniformity (i.e. a uniformity in the dose of radiation delivered to different parts of the target portion of the substrate W), which can in turn adversely affect the critical dimension uniformity (CDU).[000160] In some embodiments, the nozzle 114a, 114b is arranged to provide a gas that is directed preferentially away from the central aperture 204 in the final optical element 202. Such a gas flow is indicated schematically by arrow 222. It will be appreciated that this may be achieved, for example, if the nozzle 114a, 114b is arranged to provide the gas in the vicinity of the membrane 108 such that on the projection system PS side of the membrane 108 (i.e. in the environment 218) the gas is directed preferentially towards a peripheral portion of the gas lock funnel or towards a wall 210 of the projection system PS.[000161] In some embodiments, the nozzle 114a, 114b is arranged to provide a gas that is directed away from any optical elements in or close to a field plane of the lithographic apparatus LA. For example, the nozzle 114a, 114b may be arranged to provide the gas such that none, or a negligible amount, of the gas impinges on any optical elements in or close to a field plane of the lithographic apparatus. This is advantageous as this can avoid loss of slit uniformity and the loss of critical dimension uniformity.[000162] The final optical element 202 of the projection system PS may be an optical element of the projection system PS that radiation is incident on before being projected onto the target portion of the substrate W.[000163] In some embodiments, the nozzle 114a, 114b is arranged to provide the gas in the vicinity of the membrane 108 such that on the projection system PS side of the membrane 108 the gas is directed preferentially towards a peripheral portion of the gas lock funnel or towards a wall 210 of the projection system PS.[000164] In some embodiments, the projection system PS comprises a second optical element (in region 206) arranged such that a line extending from the gas lock funnel 102 to the central aperture 204 of the final optical element 202 intersects the second optical element.[000165] Advantageously, because the nozzle 114a, 114b is arranged to direct the gas 106a preferentially away from the central aperture 204 in the final optical element 202, the gas 106a is directed away from the second optical element, which may ensure that remnants of the membrane 108 are directed away from the second optical element in the event that the membrane 108 ruptures.[000166] The projection system PS may comprise a penultimate optical element 208. The second optical element may direct the radiation to the penultimate optical element 208 and the penultimate optical element 208 may direct the radiation to the final optical element 202.[000167] The second optical element may, for example, be disposed in or close to a field plane of the lithographic apparatus LA.[000168] In use, the lithographic apparatus LA is operable to illuminate a reticle MA (a patterning device) with a radiation beam B and to use the projection system PS to form an image of the reticle MA on a substrate W (for example a resist-coated silicon wafer) supported by the substrate table WT. That is, a plane that the reticle MA is disposed in is optically conjugate to a plane that the substrate W is disposed in. The reticle MA and substrate W planes may be referred to as field planes. As used herein, a field plane is intended to mean any plane that is optically conjugate to the reticle MA and substrate W planes.[000169] In some embodiments, the second optical element (in region 206) intersects a field plane of the lithographic apparatus LA.[000170] For such embodiments it is particularly important that a portion of the second optical element that intersects the field plane remains free from contamination. This portion may be referred to as a caustic region of the optical element.[000171] In some embodiments, the nozzle 114a, 114b is arranged to provide the gas in the vicinity of the membrane 108 such that on the projection system PS side of the membrane 108 the gas 106a is directed away from any optical elements in or close to a field plane of the lithographic apparatus LA. [000172] Any contamination on such optical elements that are in or close to a field plane is especially disadvantageous as it can negatively affect slit uniformity (i.e. a uniformity in the dose of radiation delivered to different parts of the target portion of the substrate), which can in turn adversely affect the critical dimension uniformity (CDU). An arrangement wherein the nozzle 114a, 114b is arranged to provide a gas that is directed preferentially away from any optical elements in or close to a field plane of the lithographic apparatus is advantageous as it can avoid such loss or slit uniformity and the loss of critical dimension uniformity.[000173] In some embodiments the nozzle 114a, 114b is arranged to provide the gas to an interior of the gas lock funnel 102 (for example in the vicinity of the membrane 108) such that on the projection system PS side of the gas lock funnel 102 the gas 106a is directed asymmetrically inwards towards an optical axis 104 of the lithographic apparatus LA such that the net gas flow is directed at a non-zero angle to the optical axis 104, as now discussed with reference to Figures 6A and 6B. Figures 6A and 6B are both modified versions of, and in the same plane as, the closed arrangement shown schematically in Figure 4A. The net gas flow into the projection system PS is indicated schematically in Figures 6A and 6B by arrows 124, which are at a non-zero angle to the optical axis 104.[000174] The nozzle 114a, 114b may be arranged such that the net gas flow is skewed away from the optical axis 104 (at a non-zero angle thereto) in any direction. That is, the nozzle 114a, 114b maybe arranged such that the asymmetry of the flow inwards towards the optical axis 104 may be in any direction. It will be appreciated that, in general, one particular direction may be favoured or preferable, depending on the design of the projection optics of the projection system PS.[000175] It will be appreciated that, in general, the nozzle 114a, 114b may comprise a plurality of apertures, slits or openings in a wall of the gas lock funnel 102. For example, the portions of the nozzle 114a shown in Figures 4A and 4C (or Figures 6 A and 4C or Figures 6B and 4C) are, in general, defined by separate apertures in the wall of the gas lock funnel 102. Similarly, the portions of the nozzle 114a shown on opposite sides of the optical axis 104 in any of Figures 4A, 4B, 4C, 6A and 6B are, in general, defined by separate apertures in the wall of the gas lock funnel 102.[000176] It will be further appreciated that a desired net gas flow 106a direction into the projection system PS may be achieved by selecting or designing an appropriate configuration of such apertures which define the nozzle. For example, in general, the sizes, positions and spacings of the apertures may be selected to achieve a desired net gas flow 106a direction into the projection system PS.[000177] As described above, an internal volume of the gas lock funnel 102 may be of the shape of a generalized, right frusto-conical volume with a base having a shape that is similar to that of the target portion of the substrate W. The shape of the target portion of the substrate W may be elongate (and may be curved) having a longer dimension and a shorter dimension. The asymmetry of the flow inwards towards the optical axis 104 may be in the direction of the shorter dimension (for example the y- direction, as in Figures 6A and 6B).[000178] The target portion of the substrate W may be elongate, having a longer dimension and a shorter dimension and the nozzle 114a, 114b may be arranged such that the asymmetry of the flow inwards towards the optical axis 104 is in the direction of the shorter dimension (for example the y- direction, as in Figures 6A and 6B).[000179] The lithographic apparatus LA may be a scanning lithographic apparatus and the nozzle 114a, 114b may be arranged such that the asymmetry of the flow 106a inwards towards the optical axis 104 is in a scanning direction of lithographic apparatus.[000180] In some embodiments, as shown in Figure 6 A, the nozzle 114a, 114b may be arranged such that in a first linear direction in a plane perpendicular to an optical axis 104 of the lithographic apparatus LA flow is provided in one direction only.[000181] The first linear dimension may be referred to as a y-direction. That is, the nozzle 114a, 114b is arranged to provide flow in one direction only (for example only one of the positive y-direction or the negative y-direction). The first linear dimension may be aligned with a shorter dimension of the target portion of the substrate W. The first linear dimension may be aligned with a scanning direction of lithographic apparatus LA.[000182] Alternatively, in some embodiments, as shown schematically in Figure 6B, the nozzle 114a, 114b may be arranged such that in the first linear direction in a plane perpendicular to an optical axis 104 of the lithographic apparatus LA flow is provided in two opposite directions and wherein the flowrates in the two opposite directions are not equal. This is indicated in Figure 6B by the arrows 106a indicating the gas flow from the two sides of the optical axis 104 having different lengths.[000183] In some embodiments the nozzle 114a, 114b may be arranged such that in a second linear direction in a plane perpendicular to the optical axis 104 of the lithographic apparatus LA flow is provided in two opposing directions. That is, either of the embodiments shown in Figures 6A and 6B may be combined with the arrangement shown in Figure 4C (which shows gas flow inwards from both directions in the x-direction).[000184] Such an arrangement may provide a control knob that can be used to optimize the dynamic gas lock 100. For example, this may be helpful in achieving a more optimal spatial distribution of the pressure difference across the membrane 108 and / or in case smaller gas expansion and thus lower thermal impact is preferred. In general, the combination of different segmented flow inlets above and below membrane 108 can be used as a control knob to optimize a spatial distribution of the pressure difference across the membrane 108.[000185] The second linear dimension may be generally perpendicular to the first linear dimension. The second linear dimension may be referred to as an x-direction. The second linear dimension may be aligned with a longer dimension of the target portion of the substrate W. The second linear dimension may be aligned with a non-scanning direction of lithographic apparatus LA.[000186] Alternatively, in some embodiments the nozzle may be arranged such that substantially no flow is provided in the second linear direction in a plane perpendicular to the optical axis 104 of the lithographic apparatus LA.[000187] In some embodiments the nozzle 114a, 114b may be arranged such that a gas flow 106b below the membrane 108 (i.e. in the same side as the substrate W) is substantially as shown in Figure 4B (which shows gas flow inwards from both directions in the y -direction). Alternatively, in some embodiments the nozzle 114a, 114b may be arranged such that a gas flow 106b below the membrane 108 (i.e. in the same side as the substrate W) is skewed in a similar way to the skew of the flow of gas 106a above the membrane 108.[000188] In some embodiments, the nozzle 114a, 114b may be arranged to provide the gas in the vicinity of the membrane 108 such that towards the projection system (for example on the projection system PS side of the membrane 108 when present) the gas 106a is directed so as to swirl about an optical axis 104 of the lithographic apparatus LA, as shown schematically in Figure 7.[000189] The nozzle 114a, 114b may comprise one or more apertures shaped so as to impart swirl to the gas on the projection system PS side of the membrane 108. The apertures may, for example, impart the swirl to the gas according to the Coanda effect.[000190] In some embodiments, the nozzle 114a, 114b may comprise one or more guide vanes shaped so as to impart swirl to the gas on the projection system PS side of the membrane 108. The guide vanes may, for example, extend across an exit opening or slit. The guide vanes may act as aerofoils to direct the flow 106a to swirl around the optical axis 104.[000191] In some embodiments, the nozzle 114a, 114b may comprise an opening on a wall of the gas lock funnel 102, wherein said opening extends around only a portion of a perimeter of the gas lock funnel 102. The opening may be a slit that extends around a portion of an axis 104 of the gas lock funnel 102.[000192] The gas lock funnel 102 may comprise a seal shield 116 having inlets and / or nozzles arranged to provide a gas to the opening.[000193] In some embodiments, the lithographic apparatus LA further comprises a debris catcher arranged to catch membrane 108 fragments in the event of rupture of the membrane 108. The debris catch may be disposed proximate to the membrane 108 on the projection system PS side thereof. Additionally or alternatively, the debris catch may be disposed in or proximate to the central aperture 204 of a last optical element 202 in the projection system PS.[000194] The debris catch may comprise an electrostatic device that is configured to attract debris towards it. For such embodiments, the debris catch may be disposed in or proximate to the central aperture 204 of a last optical element 202 in the projection system PS.[000195] Additionally or alternatively, the debris catch may comprise a net of mesh structure, which may, for example, comprise a carbon nanotube membrane. For such embodiments, the debris catch may be disposed proximate to the membrane 108 to facilitate removal of the debris.[000196] In some embodiments, the lithographic apparatus LA may further comprise a gas source operable to deliver gas to the nozzle 114a, 114b. The gas may, for example, be hydrogen gas.[000197] Some embodiments of the present disclosure relate to an assembly for use in a lithographic apparatus LA, the assembly comprising the gas lock funnel 102. Optionally, the assembly may further comprise the membrane 108 supported by the gas lock funnel 102, as described above.[000198] The nozzle 114a, 114b may be arranged such that the net gas flow 106a is skewed away from an axis of the gas lock funnel 102 (at a non-zero angle thereto) (as discussed above with reference to Figures 6A and 6B). The nozzle 114a, 114b may be arranged such that the asymmetry of the flow inwards towards the axis may be in any direction. It will be appreciated that, in general, one particular direction may be favoured or preferable, depending on the design of the projection optics of the projection system PS of the lithographic apparatus LA that the assembly is used in.[000199] A cross sectional shape of the gas lock funnel 102 in a plane perpendicular to its axis 104 may be elongate, having a longer dimension and a shorter dimension. The nozzle 114a, 114b may be arranged such that the asymmetry of the flow 106a inwards towards the axis 104 is in the direction of the shorter dimension.[000200] The nozzle may be arranged such that in a first linear direction in a plane perpendicular to an axis 104 of the gas lock funnel 102 flow is provided in one direction only.[000201] The first linear dimension may be referred to as a y-direction. That is, the nozzle 114a, 114b is arranged to provide flow that is in one direction only (for example only one of the positive y- direction or the negative y-direction). The first linear dimension may be aligned with a shorterdimension of the cross sectional shape of the gas lock funnel 102 in a plane perpendicular to its axis 104. In use, the first linear dimension may be aligned with a scanning direction of lithographic apparatus LA.[000202] Alternatively, in some embodiments the nozzle 114a, 114b may be arranged such that in the first linear direction in a plane perpendicular to the axis 104 of the gas lock funnel 102 flow is provided in two opposite directions and wherein the flow rates in the two opposite directions are not equal.[000203] The nozzle 114a, 114b may be arranged such that in a second linear direction in a plane perpendicular to the axis 104 of the gas lock funnel 102 flow is provided in two opposing directions.[000204] Such an arrangement may provide a control knob that can be used to optimize the dynamic gas lock 100. For example, this may be helpful in achieving a more optimal spatial distribution of the pressure difference across the membrane 108 and / or in case smaller gas expansion and thus lower thermal impact is preferred. In general the combination of different segmented flow inlets above and below membrane 108 can be used as a control knob to optimize a spatial distribution of the pressure difference across the membrane 108.[000205] The second linear dimension may be generally perpendicular to the first linear dimension. The second linear dimension may be referred to as an x-direction. The second linear dimension may be aligned with a longer dimension of the cross sectional shape of the gas lock funnel 102 in a plane perpendicular to its axis 104. In use, the second linear dimension may be aligned with a non-scanning direction of lithographic apparatus LA.[000206] Alternatively, in some embodiments the nozzle 114a, 114b may be arranged such that substantially no flow is provided in the second linear direction in a plane perpendicular to the axis 104 of the gas lock funnel 102.[000207] In some embodiments, the nozzle 114a, 114b may be arranged to provide the gas in the vicinity of the membrane such that on a first side of the membrane 108 the gas is directed so as to swirl about an axis 104 of the gas lock funnel 102.[000208] In some embodiments, the nozzle 114a, 114b may comprise one or more apertures shaped so as to impart swirl to the gas on a first side of the membrane 108 (as discussed above with reference to Figure 7). The apertures may, for example, impart the swirl to the gas according to the Coanda effect. [000209] In some embodiments, the nozzle 114a, 114b may comprise one or more guide vanes shaped so as to impart swirl to the gas on a first side of the membrane 108. The guide vanes may, for example, extend across an exit opening or slit. The guide vanes may act as aerofoils arranged to direct the flow 106a.[000210] In some embodiments, the nozzle 114a, 114b may comprise an opening on a wall of the gas lock funnel 102, wherein said opening extends around only a portion of a perimeter of the gas lock funnel 102. The opening may be a slit that extends around a portion of an axis 104 of the gas lock funnel 102.[000211] It will be appreciated that, in use, one or more interior walls of the gas lock funnel 102 may form an extension of one or more walls 210, 212, 214 that define part of an interior of the projection system PS. Therefore, any reference herein to a gas flow being directed preferentially towards a peripheral portion (or wall) of the gas lock funnel 102 may include arrangements wherein the gas is directed preferentially towards a wall 210, 212, 214 of the projection system PS.[000212] Embodiments of the present disclosure relating to such a dynamic gas lock in the vicinity of the opening 8 in an enclosing structure 9 of the radiation source SO, where the radiation beam B exits the radiation source and enters the illumination system IL, are now described with reference to Figures 8A to 9B. Figure 8A schematically shows a known type of a dynamic gas lock 300 in the vicinity of the opening 8 in an enclosing structure 9 of the radiation source SO. Figure 8B shows an enlarged portion of the known arrangement of Figure 8A. Figure 9A shows an enlarged portion of a first embodiment of a new arrangement for a dynamic gas lock 400 in the vicinity of the opening 8 in an enclosing structure 9 of the radiation source SO according to the present disclosure. Figure 9B shows an enlarged portion of a second embodiment of a new arrangement for a dynamic gas lock 400 in the vicinity of the opening 8 in an enclosing structure 9 of the radiation source SO according to the present disclosure.[000213] Such a known dynamic gas lock 300 is indicated schematically in Figures 8A and 8B and is used to limit tin debris exiting the radiation source SO and entering the lithographic apparatus LA. It is a flow that is injected at a differential pumping interface between the radiation source SO and lithographic apparatus (in the vicinity of the opening 8 in the enclosure 9 of the radiation source SO). The dynamic gas lock 300 comprises one or more nozzles which are arranged to provide both: (i) a flow of gas 310 into the radiation source SO; and (ii) a flow of gas 320 into the lithographic apparatus LA. These gas flows 310, 320 may comprise jets that are supersonic or near supersonic in nature.[000214] The flow of gas 310 into the radiation source SO (which comprises a high speed jet) reduces the number of tin particles that pass through the opening 8 from the radiation source SO to the lithographic apparatus LA. Tin contamination in the lithographic apparatus LA can lead to two significant problems: (a) a loss of transmission of optics in the lithographic apparatus LA, directly reducing lithographic throughput; and (b) defects on the patterning device MA (reticle) that can lead to wafer die yield loss. It is believed that known dynamic gas locks are suitable to block or hinder Tin particles originating from the radiation source SO up to a certain size. Such particles may e.g. evaporate when interacting with the radiation beam emitted by the radiation source SO through the opening 8. Such particles may thus be blocked from reaching the illumination system IL. Particles of a comparatively large size may however be difficult to block or hinder by the dynamic gas lock. Such particles may e.g. undergo a (partial) evaporation when interacting with the radiation beam and arrive downstream of the opening 8. These particles, which are then reduced in size, may e.g. propagate towards optical components of the illumination system or the patterning device causing the aforementioned issues.[000215] In existing dynamic gas locks (see Figures 8A and 8B) the two dynamic gas lock flows 310, 320 are symmetric and generally aligned with the optical axis 340 of the system. As a result, the flow of gas 320 into lithographic apparatus LA points directly at an entrance aperture 350 of the illumination system IL.[000216] However, there is a significant pressure difference between the source vessel 9 and the illumination system IL. The inventors have realized that, as a result of this pressure difference, any particles that make it through the aperture 8 from the radiation source SO to the illumination system IL experiences a substantial acceleration as it enters the gas flow 320 into the illumination system IL. Furthermore, once accelerated, such particles enter into a relatively low pressure environment (and thus experience relatively low drag force) and travels along a similar path to the radiation beam B.[000217] In the schematic illustration shown in Figure 8B, the known arrangement comprises two rows of apertures (two apertures from each row are visible in the cross section of Figure 8B) at different axial positions. A first row of apertures is in fluid communication with a first manifold or inlet 360 and generally supplies the flow of gas 310 into the radiation source SO. A second row of apertures is in fluid communication with a second manifold or inlet 362 and generally supplies the flow of gas 320 into the lithographic apparatus LA.[000218] As shown in Figures 9A and 9B, the new arrangement for a dynamic gas lock 400 in the vicinity of the opening 8 in an enclosing structure 9 of the radiation source SO according to an embodiment of the present disclosure is arranged to provide a gas to an interior of the gas lock funnel such that on the illumination system IL side of the gas lock funnel the gas 320 is directed preferentially away from optical elements of the illumination system IL.[000219] In particular, the new arrangement for a dynamic gas lock 400 in the vicinity of the opening 8 in an enclosing structure 9 of the radiation source SO according to an embodiment of the present disclosure is arranged to provide the gas to the interior of the gas lock funnel such that on the illumination system IL side of the gas lock funnel the gas is directed asymmetrically inwards towards an optical axis of the lithographic apparatus such that the net gas flow 320 is directed at a non-zero angle to the optical axis 340.[000220] It will be appreciated that the gas 320 being directed preferentially away from optical elements of the illumination system IL is intended to mean that at least a majority of the gas 320 is directed towards something which is not used to direct the radiation B (towards a substrate W or a patterning device MA). That is, preferably at least 50% of the gas 320 may be directed so as not to impinge on the optical elements of the illumination system IL, more preferably at least 75% and even more preferable at least 85% of the gas 320 may be directed so as not to impinge on the optical elements of the illumination system IL. Optical elements of the illumination system IL may include, for example, mirrors, sensors and so on.[000221] The illuminator system IL may be considered to be an optical system and the radiation source SO may be considered to be a second system.[000222] The illumination system IL comprises a housing in which the optics 10, 11 are disposed. The housing defines the aperture 350 which, in use, the radiation B propagates through. The nozzle of the dynamic gas lock 400 is arranged to provide the gas to the interior of the gas lock funnel such that on the illumination system IL side of the gas lock funnel a net direction of the gas 320 does not point towards the aperture 350.[000223] It will be appreciated that as used here, the net direction of the gas 320 not pointing towards the aperture 350 is intended to mean that if a line or arrow 322 pointing in the direction of the net direction of the gas 320 was extended towards the illumination system IL it would not intersect with or pass through the aperture 350. Rather, it would intersect with a wall of the housing.[000224] In order to facilitate such embodiments, a shape of the gas lock funnel may be altered relative to known arrangements. For example, the gas lock funnel may define a volume, a shape of which is generally of the form of a right frusto-conical volume, which defines the axis of the gas lock funnel. In order to ensure that a net direction of the gas flow 320 on the illumination system IL side does not point towards the aperture 350, a divergence or tapering of the gas lock funnel may be increased. This increase in the divergence or tapering of the gas lock funnel can be seen when comparing either Figure 9A or Figure 9B with Figure 8B.[000225] In some embodiments, the nozzle comprises a plurality of apertures defined in a wall of the gas lock funnel, the plurality of apertures being circumferentially spaced around an axis of the gas lock funnel. For example, there may be of the order of 12 or 13 apertures circumferentially distributed around the axis of the gas lock funnel (only one such aperture can be seen in the cross section of Figure 9A and only two such apertures can be seen in the cross section of Figure 9B).[000226] In some embodiments, the nozzle may comprises more than one (for example two or three) rows of apertures defined in a wall of the gas lock funnel and circumferentially spaced around an axis of the gas lock funnel.[000227] As schematically illustrated in Figure 9A, the plurality of apertures defined in the wall of the gas lock funnel may be asymmetrically arranged around the axis 340 of the gas lock funnel. For example, by providing more apertures on a first circumferential portion (right-hand side of Figure 9A) than a second circumferential portion (left-hand side of Figure 9A), a net direction of the gas flow 320 may be tilted away from an axis 340 of the gas lock funnel towards the second circumferential portion (left-hand side of Figure 9A). In order to retrofit such an arrangement to existing gas lock funnels, one or more existing apertures on the second circumferential portion may be blocked.[000228] As schematically illustrated in Figure 9B, the plurality of apertures defined in the wall of the gas lock funnel may comprise at least two sets of apertures, each set may have a different manifold or inlet. In the schematic example shown in Figure 9B, set separate manifolds 462, 464 are shown (each servicing a different set of apertures). Each set of apertures comprises at least one of the plurality of apertures. Each set of apertures is provided on a different circumferential portion of the gas lock funnel. The apertures of any given set may share a common inlet or manifold.[000229] This arrangement (shown in Figure 9B) may provide a more controllable and versatile arrangement for controlling a net direction of the gas flow 320. For example, by independently controlling a flow rate provided to the manifolds 462, 464 of different sets of apertures, a net direction of the gas flow 320 may be controlled. Such an arrangement (as shown in Figure 9B) may also allow the net direction of the gas flow 320 to be either off-axis (as described above) or on-axis (i.e. similar to the known arrangement shown in Figures 8A and 8B). Furthermore, the new dynamic gas lock 400 may be switched between such off-axis and on-axis configurations by independently controlling a flow rate provided to the manifolds 462, 464 of different sets of apertures.[000230] In general, the number, sizes and distribution of apertures of the nozzle can be selected so as to control the net direction of the flow of gas 320 into the lithographic apparatus LA.[000231] The new dynamic gas lock 400 prevents the gas flow 320 from entering into a housing of the illumination system IL by directing it away from optical axis 340 and passing around the housing of the illumination system IL to an exhaust. Such arrangements do not impose any changes to the gas budget in wither the radiation source SO or the lithographic apparatus LA.[000232] In some embodiments, some type of tin capture structure may be added to manage the tin particles diverted away from the entrance of the illumination system IL. Optionally, if an existing nearby exhaust is available, this may be used to manage such tin particles.[000233] Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquidcrystal displays (LCDs), thin-film magnetic heads, etc.[000234] Although specific reference may be made in this text to embodiments in the context of a lithographic apparatus, embodiments may be used in other apparatus. Embodiments may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrate) or mask (or other patterning device). These apparatus may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non-vacuum) conditions.[000235] Although specific reference may have been made above to the use of embodiments in the context of optical lithography, it will be appreciated that the invention, where the context allows, is not limited to optical lithography and may be used in other applications, for example imprint lithography.[000236] Where the context allows, embodiments may be implemented in hardware, firmware, software, or any combination thereof. Embodiments may also be implemented as instructions stored on a machine-readable medium, which may be read and executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine-readable medium may include read only memory (ROM); random access memory (RAM); magnetic storage media; opticalstorage media; flash memory devices; electrical, optical, acoustical or other forms of propagated signals (e.g. carrier waves, infrared signals, digital signals, etc.), and others. Further, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions in fact result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc. and in doing that may cause actuators or other devices to interact with the physical world.[000237] The invention may also be formulated as clauses:1. A lithographic system comprising: an optical system comprising optics configured to project radiation onto a target portion of a substrate or to project radiation onto a patterning device; a second system; and a gas lock funnel arranged between the optical system and the second system; wherein the gas lock funnel defines a nozzle that is arranged to provide a gas to an interior of the gas lock funnel such that on the optical system side of the gas lock funnel the gas is directed preferentially away from optical elements of the optical system.2. The lithographic system of clause 1 wherein the second system comprises a substrate table configured to hold a substrate, and wherein the optical system comprises a projection system comprising optics configured to project radiation onto the target portion of the substrate.3. The lithographic system of clause 1 wherein the second system comprises a radiation source, and wherein the optical system comprises an illuminator system configured to project radiation onto a patterning device.4. A lithographic apparatus comprising: a substrate table configured to hold a substrate; a projection system comprising optics configured to project radiation onto a target portion of the substrate; and a gas lock funnel arranged between the projection system and the substrate table; wherein a final optical element of the projection system has a central aperture; and wherein the gas lock funnel defines a nozzle that is arranged to provide a gas to an interior of the gas lock funnel such that on the projection system side of the gas lock funnel the gas is directed preferentially away from the central aperture in the final optical element.5. The lithographic apparatus of clause 4 wherein the nozzle is arranged to provide the gas to the interior of the gas lock funnel such that on the projection system side of the gas lock funnel the gas is directed preferentially away from optical elements of the projection system.6. The lithographic apparatus of clause 4 or clause 5 wherein the projection system comprises a second optical element arranged such that a line extending from the gas lock funnel to the central aperture of the final optical element intersects the second optical element.7. The lithographic apparatus of clause 6 wherein the second optical element intersects a field plane of the lithographic apparatus.8. The lithographic apparatus or lithographic system of any preceding clause, further comprising a membrane supported by the gas lock funnel and arranged between the projection system or the optical system and the substrate table or the second system and wherein the nozzle of the gas lock funnel is arranged to provide the gas in the vicinity of the membrane.9. The lithographic apparatus or lithographic system of any preceding clause wherein the nozzle is arranged to provide the gas such that on the projection system or optical system side of the gas lock funnel the gas is directed away from any optical elements in or close to a field plane of the lithographic apparatus.10. The lithographic apparatus or lithographic system of any preceding clause wherein the nozzle is arranged to provide the gas to the interior of the gas lock funnel such that on the projection system or optical system side of the gas lock funnel the gas is directed asymmetrically inwards towards an optical axis of the lithographic apparatus such that the net gas flow is directed at a non-zero angle to the optical axis.11. The lithographic apparatus or lithographic system of clause 10 when dependent either directly or indirectly on clause 2 or clause 4 wherein the target portion of the substrate is elongate, having a longer dimension and a shorter dimension, and wherein the nozzle is arranged such that the asymmetry of the flow inwards towards the optical axis is in the direction of the shorter dimension.12. The lithographic apparatus or lithographic system of clause 10 or clause 11 wherein the lithographic apparatus is a scanning lithographic apparatus and wherein the nozzle is arranged such that the asymmetry of the flow inwards towards the optical axis is in a scanning direction of the lithographic apparatus.13. The lithographic apparatus or lithographic system of any one of clauses 10 to 12 wherein the nozzle is arranged such that in a first linear direction in a plane perpendicular to an optical axis of the lithographic apparatus flow is provided in one direction only.14. The lithographic apparatus or lithographic system of clause 13 wherein the nozzle is arranged such that in a second linear direction in a plane perpendicular to the optical axis of the lithographic apparatus flow is provided in two opposing directions.15. The lithographic apparatus or lithographic system of any preceding clause wherein the nozzle is arranged to provide the gas to the interior of the gas lock funnel such that on the projection system or optical system side of the gas lock funnel the gas is directed so as to swirl about an optical axis of the lithographic apparatus.16. The lithographic apparatus or lithographic system of clause 15 wherein the nozzle comprises one or more apertures shaped so as to impart swirl to the gas on the projection system or optical system side of the gas lock funnel.17. The lithographic apparatus or lithographic system of clause 15 or clause 16 wherein the nozzle comprises one or more guide vanes shaped so as to impart swirl to the gas on the projection system or optical system side of the gas lock funnel.18. The lithographic apparatus or lithographic system of any preceding clause, wherein the nozzle comprises an opening on a wall of the gas lock funnel, wherein said opening extends around only a portion of a perimeter of the gas lock funnel.19. The lithographic apparatus or lithographic system of clause 18 wherein the gas lock funnel comprises a seal shield having inlets and / or nozzles arranged to provide a gas to the opening.20. The lithographic apparatus or lithographic system of any preceding clause further comprising a debris catcher arranged to catch debris.21. The lithographic apparatus or lithographic system of any preceding clause further comprising a gas source operable to deliver gas to the nozzle.22. The lithographic apparatus or lithographic system of any preceding clause wherein the projection system or optical system comprises a housing in which the optics are disposed, wherein the housing defines an aperture which, in use, the radiation propagates through and wherein the nozzle is arranged to provide the gas to the interior of the gas lock funnel such that on the projection system or optical system side of the gas lock funnel a net direction of the gas does not point towards the aperture.23. The lithographic apparatus or lithographic system of any preceding clause wherein the nozzle comprises a plurality of apertures defined in a wall of the gas lock funnel, the plurality of apertures being circumferentially spaced around an axis of the gas lock funnel.24. The lithographic apparatus or lithographic system of clause 23 wherein the plurality of apertures defined in the wall of the gas lock funnel are asymmetrically arranged around the axis of the gas lock funnel.25. The lithographic apparatus or lithographic system of clause 23 or clause 24 wherein the plurality of apertures defined in the wall of the gas lock funnel comprise at least two sets of apertures, each set of apertures comprising at least one of the plurality of apertures, each set of apertures being provided on a different circumferential portion of the gas lock funnel, wherein the apertures of any given set share a common inlet or manifold and wherein each set has a different manifold or inlet.26. An assembly for use in the lithographic apparatus or lithographic system of any preceding clause, the assembly comprising the gas lock funnel; and the membrane supported by the gas lock funnel.27. An assembly for use in a lithographic apparatus or lithographic system, the assembly comprising: a gas lock funnel; wherein the gas lock funnel defines a nozzle that is arranged to provide a gas in to an interior of the gas lock funnel such that on a first side of the gas lock funnel the gas is directed preferentially towards a peripheral portion of the gas lock funnel.28. The assembly of clause 27 further comprising a membrane supported by the gas lock funnel.29. The assembly of clause 27 or clause 28 wherein the nozzle is arranged such that the gas is not directed along an axis of the gas lock funnel.30. The assembly of clause 29 wherein the gas lock funnel defines a volume, a shape of which is generally of the form of a generalized, right frusto-conical volume, which defines the axis of the gas lock funnel.31. The assembly of any one of clauses 27 to 30 wherein the nozzle is arranged to provide the gas to the interior of the gas lock funnel such that on the first side of the gas lock funnel the gas is directed asymmetrically inwards towards an axis of the gas lock funnel such that the net gas flow is directed at a non-zero angle to the axis.32. The assembly of clause 31 wherein a cross sectional shape of the gas lock funnel in a plane perpendicular to its axis is elongate, having a longer dimension and a shorter dimension, and wherein the nozzle is arranged such that the asymmetry of the flow inwards towards the axis is in the direction of the shorter dimension.33. The assembly any one of clauses 27 to 32 wherein the nozzle is arranged such that in a first linear direction in a plane perpendicular to an axis of the gas lock funnel the flow is provided in one direction only.34. The assembly of clause 33 wherein the nozzle is arranged such that in a second linear direction in a plane perpendicular to the axis of the gas lock funnel gas flow is provided in two opposing directions.35. The assembly of any one of clauses 27 to 34 wherein the nozzle is arranged to provide the gas to the interior of the gas lock funnel such that on the first side of the gas lock funnel the gas is directed so as to swirl about an axis of the gas lock funnel.36. The assembly of clause 35 wherein the nozzle comprises one or more apertures shaped so as to impart swirl to the gas on the first side of the membrane.37. The assembly of clause 35or clause 36 wherein the nozzle comprises one or more guide vanes shaped so as to impart swirl to the gas on the first side of the membrane.38. The assembly of any one of clauses 27 to 37 wherein the nozzle comprises an opening on a wall of the gas lock funnel, wherein said opening extends around only a portion of a perimeter of the gas lock funnel.39. The assembly of any one of clauses 27 to 38 wherein the nozzle comprises a plurality of apertures defined in a wall of the gas lock funnel, the plurality of apertures being circumferentially spaced around an axis of the gas lock funnel.40. The assembly of clause 39 wherein the plurality of apertures defined in the wall of the gas lock funnel are asymmetrically arranged around the axis of the gas lock funnel.41. The assembly of clause 39 or clause 40 wherein the plurality of apertures defined in the wall of the gas lock funnel comprise at least two sets of apertures, each set of apertures comprising at least oneof the plurality of apertures, each set of apertures being provided on a different circumferential portion of the gas lock funnel, wherein the apertures of any given set share a common inlet or manifold and wherein each set has a different manifold or inlet.While specific embodiments have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.

Claims

CLAIMS1. A lithographic system comprising: an optical system comprising optics configured to project radiation onto a target portion of a substrate or to project radiation onto a patterning device; a second system; and a gas lock funnel arranged between the optical system and the second system; wherein the gas lock funnel defines a nozzle that is arranged to provide a gas to an interior of the gas lock funnel such that on the optical system side of the gas lock funnel the gas is directed preferentially away from optical elements of the optical system.

2. The lithographic system of claim 1 wherein the second system comprises a substrate table configured to hold a substrate, and wherein the optical system comprises a projection system comprising optics configured to project radiation onto the target portion of the substrate.

3. The lithographic system of claim 1 wherein the second system comprises a radiation source, and wherein the optical system comprises an illuminator system configured to project radiation onto a patterning device.

4. A lithographic apparatus comprising: a substrate table configured to hold a substrate; a projection system comprising optics configured to project radiation onto a target portion of the substrate; and a gas lock funnel arranged between the projection system and the substrate table; wherein a final optical element of the projection system has a central aperture; and wherein the gas lock funnel defines a nozzle that is arranged to provide a gas to an interior of the gas lock funnel such that on the projection system side of the gas lock funnel the gas is directed preferentially away from the central aperture in the final optical element.

5. The lithographic apparatus of claim 4 wherein the nozzle is arranged to provide the gas to the interior of the gas lock funnel such that on the projection system side of the gas lock funnel the gas is directed preferentially away from optical elements of the projection system.

6. The lithographic apparatus of claim 4 or claim 5 wherein the projection system comprises a second optical element arranged such that a line extending from the gas lock funnel to the central aperture of the final optical element intersects the second optical element.

7. The lithographic apparatus of claim 6 wherein the second optical element intersects a field plane of the lithographic apparatus.

8. The lithographic apparatus or lithographic system of any preceding claim, further comprising a membrane supported by the gas lock funnel and arranged between the projection system or the optical system and the substrate table or the second system and wherein the nozzle of the gas lock funnel is arranged to provide the gas in the vicinity of the membrane.

9. The lithographic apparatus or lithographic system of any preceding claim wherein the nozzle is arranged to provide the gas such that on the projection system or optical system side of the gas lock funnel the gas is directed away from any optical elements in or close to a field plane of the lithographic apparatus.

10. The lithographic apparatus or lithographic system of any preceding claim wherein the nozzle is arranged to provide the gas to the interior of the gas lock funnel such that on the projection system or optical system side of the gas lock funnel the gas is directed asymmetrically inwards towards an optical axis of the lithographic apparatus such that the net gas flow is directed at a non-zero angle to the optical axis.

11. The lithographic apparatus or lithographic system of claim 10 when dependent either directly or indirectly on claim 2 or claim 4 wherein the target portion of the substrate is elongate, having a longer dimension and a shorter dimension, and wherein the nozzle is arranged such that the asymmetry of the flow inwards towards the optical axis is in the direction of the shorter dimension.

12. The lithographic apparatus or lithographic system of any preceding claim wherein the nozzle is arranged to provide the gas to the interior of the gas lock funnel such that on the projection system or optical system side of the gas lock funnel the gas is directed so as to swirl about an optical axis of the lithographic apparatus.

13. The lithographic apparatus or lithographic system of claim 12 wherein the nozzle comprises one or more apertures shaped so as to impart swirl to the gas on the projection system or optical system side of the gas lock funnel.

14. The lithographic apparatus or lithographic system of claim 12 or claim 13 wherein the nozzle comprises one or more guide vanes shaped so as to impart swirl to the gas on the projection system or optical system side of the gas lock funnel.

15. The lithographic apparatus or lithographic system of any preceding claim, wherein the nozzle comprises an opening on a wall of the gas lock funnel, wherein said opening extends around only a portion of a perimeter of the gas lock funnel.

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